The present invention relates to a display apparatus using electroluminescence elements, and a method of manufacturing the display apparatus.
Electroluminescence (EL) elements for use in a display apparatus include an inorganic EL element and an organic EL element. The inorganic EL element uses a thin film of an inorganic compound, like zinc selenide or zinc sulfide, as a fluorescent material, and the organic EL element uses an organic compound as a fluorescent material. Preferably, the organic EL element has the following features:
(1) A high external quantum efficiency. PA1 (2) Light is emitted on a low driving voltage. PA1 (3) Multifarious colors (green, red, blue, yellow, etc.) can be generated by selecting a proper fluorescent material. PA1 (4) The display is clear and no back light is needed. PA1 (5) There is no dependency on the viewing angle. PA1 (6) The organic EL element is thin and light. PA1 (7) A soft material like a plastic film can be used for the substrate.
Due to the aforementioned features, a display apparatus using such an organic EL elements (hereinafter referred to as "organic EL display apparatus") is a desirable replacement for a CRT or liquid crystal display.
An organic EL display apparatus employs a dot matrix system which displays an image with dots arranged in a matrix form. The dot matrix system includes a simple matrix system or an active matrix system.
The simple matrix system directly drives organic EL elements of a matrix of pixels, arranged on a display panel, in synchronism with a scan signal using an external driving unit. Each pixel on the display panel has only an organic EL element. As the number of scan lines of a display apparatus increases, therefore, the driving time (duty) assigned to each pixel decreases. This reduces the contrast as well as the luminance intensity of the display screen.
In the active matrix system, each of pixels arranged in a matrix form has an organic EL element and a pixel driving element (active elements). The pixel driving element serves as a switch which is switched on or off by the scan signal. A data signal (display signal, video signal) is transmitted to the anode of the associated organic EL element via an enabled pixel driving element. As the data signal is written in the organic EL element, the organic EL element is driven. Even when the pixel driving element is switch off state, the data signal previously applied to the anode of the organic EL element is held, in the form of a charge, in the organic EL element. The organic EL element is kept driven until the associated pixel driving element is switched on again. Even if the driving time per pixel driving element decreases as the number of scan lines is increased, therefore, the driving of the organic EL elements is not affected. Specifically, a sufficient luminance intensity is secured for an image to be displayed on the display panel and reduction in the contrast is prevented. The active matrix system can therefore provide display images with a higher quality than the simple matrix system.
Depending on the difference in pixel driving elements, display apparatuses of the active matrix system are classified into a transistor type (three-terminal type) and a diode type (two-terminal type). The transistor type is more difficult to manufacture than the diode type. On the other hand, the contrast and resolution of images which are displayed by the transistor type display apparatus can be improved easily. Therefore, the transistor type display apparatus provides high-quality images which match with those displayed by a CRT display. The following description on the operational principle of the active matrix system is mainly associated with the transistor type.
A conventional organic EL display apparatus 101 of the simple matrix system will now be discussed with reference to FIGS. 1 through 3.
As shown in FIG. 1, a plurality of anodes 103 are arranged, parallel to one another, on an insulator substrate 102, and a light emitting layer 104 is provided on the insulator substrate 102 to cover the anodes 103. A plurality of cathodes 105 are arranged, parallel to one another, on the light emitting layer 104. The anodes 103 are placed perpendicular to the cathodes 105. The light emitting layer 104, the anodes 103 and the cathodes 105 form an organic EL element 106. The insulator substrate 102 is preferably made of transparent glass, synthetic resin or the like. The anodes 103 are preferably formed of transparent electrodes of ITO (Indium Tin Oxide) or the like. The light emitting layer 104 is preferably formed of an organic compound. The cathodes 105 are preferably formed of a magnesium-indium alloy.
In the organic EL element 106, holes coming from the anodes 103 are recombined with electrons coming from the cathodes 105 inside the light emitting layer 104, emitting light. The light is emitted outside via the anodes 103 and the transparent insulator substrate 102 as indicated by the arrow .gamma. in FIG. 2.
FIG. 3 is a schematic plan view of the organic EL display apparatus 101, as viewed from the anodes 103. In FIG. 3, only the anodes 103 and the cathodes 105 are illustrated.
Defined at the individual intersections of anodes 103a to 103c and cathodes 105a to 105c are light emitting areas B1 to B9 which emit light, as discussed above. That is, the light emitting areas B1-B9, arranged in a matrix form, form the organic EL display apparatus 101.
In the simple matrix system, the positive terminal of a driving power supply is connected to one anode 103, and the negative terminal of the driving power supply is connected to the corresponding cathode 105. In this manner, the anode 103 and the cathode 105 are energized.
In order for light emitting area B2 at the intersection of the anode 103b and the cathode 105a to emit light, for example, the positive terminal is connected to the anode 103b and the negative terminal is connected to the cathode 105a, and power is supplied through the terminals. As a result, a forward current flows, as indicated by the arrow .gamma..
Since the light emitting layer 104 is provided on the insulator substrate 102 so as to cover a plurality of anodes 103, a leak current flows, as indicated by the arrow .beta.. The leak current energizes not only the light emitting area B2, but also the light emitting areas B1, B3 and B5 near the light emitting area B2. As a result, the light emitting areas B1, B3 and B5 emit light. This phenomenon is called an optical crosstalk caused by the leak current characteristic of the EL element 106. This shortcoming is inherent not only to the simple matrix system but also to the active matrix system and occurs in an inorganic EL display apparatus as well as an organic EL display apparatus.
The optical crosstalk deteriorates the contrast of images displayed by the organic EL display apparatus 101, disabling the acquisition of high-definition images. Particularly, a full-color organic EL display apparatus using EL elements causes color "smearing" and does not provide clear images.
Japanese Unexamined Patent Publication No. Hei 4-249095 discloses an EL element which prevents the occurrence of such a crosstalk as a solution to the aforementioned problem. In the EL element, a plurality of light emitting elements each comprised of an organic compound are provided apart from one another between a transparent electrode and a plurality of metal electrodes. Since the individual light emitting elements in the EL element are provided apart from one another, a crosstalk originating from the leak current does not occur. This makes an image to be displayed clearer.
As light emitting elements are made of an organic compound, they have poor water resistance. Therefore, the photolithography technology which uses cleaning water cannot be used in separating light emitting elements. The individual EL elements are thus formed directly using a metal mask. The use of the metal mask restricts the micro work of light emitting elements and gets in the way of improving the clearness of display images.
Accordingly, it is an object of the present invention to provide a display apparatus capable of displaying clear images and a method of manufacturing the same.